1. Field of the Invention
This invention relates to a capillary and procedure for bonding wire to a bonding surface, particularly in conjunction with semiconductor devices.
2. Brief Description of the Prior Art
In the fabrication of semiconductor devices, wiring between two bond pads on a chip or between a bond pad on the chip and an external bonding location, such as a lead frame, is generally performed by making a ball bond on one bonding pad at one end of a wire fed from a spool and then making a stitch bond at the other end of the wire. The wire is also severed from the spool from which it has been fed at the stitch bond location. Such bonding and wire severing is generally performed with the aid of a capillary through which the wire to be bonded is passed from the wire spool. The procedure is to form a ball from a portion of the wire which extends out of the capillary, bond the ball to a bond pad and move the capillary to the second bond pad with the wire being fed out through the capillary during travel of the capillary to the second bond pad. The wire is then stitch bonded to the second pad, usually a lead finger, and prepared for severing from the spool, using the nose of the capillary to perform these tasks.
In order for the capillary to perform the function of stitch bonding, the capillary must apply a force against the wire which rests on the bond pad. In order to perform this task without fracture of the capillary, as a first condition, the capillary wall must have sufficient thickness to withstand the forces applied thereto at the location of force application during the stitch bonding procedure. This has been accomplished in the prior art by providing a wire bonding capillary with central bore or aperture, a circular cross section and wall thickness from exterior surface to central bore sufficient to accomplish the above described purpose and withstand the forces thereon.
With the continual decrease in the spacing dimensions between wire bonding locations, particularly in the semiconductor art, the problem of bonding wire to a wire bonding surface with a capillary and then moving the capillary to a new location without interference with adjacent wire bonding locations and wires bonded at adjacent wire bonding locations has increased. As the dimensions decrease, the possibility that the capillary will interfere with or strike an adjacent bond pad or wire extending from an adjacent bond pad in its travel from one bonding location to a second bonding location increases.
A prior art technique that has been developed to accommodate and/or minimize this problem has utilized a capillary with the diameter of the nose portion decreased. This results in a reduction of capillary wall thickness and provides a poor stitch bond. Accordingly, this technique is undesirable.
A further prior art technique that has been developed to accommodate and/or minimize this problem has utilized a wire bonding capillary of circular cross section with a portion of the capillary wall on a pair of opposing sides of the capillary removed to provide an indentation thereat. Capillaries of this type are provided by Texas Instruments Incorporated under the trademark BowTI.TM.. This is accomplished by using a capillary having a nose or stitch face somewhat in the shape of a figure "8" with a hollow center to carry the wire as in the prior art and with an enlarged but thin walled waist region. A capillary with this shape is still capable of performing the functions of forming and bonding the ball from the wire passing therethrough at one pad and then stitch bonding the wire at a second pad, using the thicker-walled portion of the "8"-shaped capillary. The top and bottom portions of the "8" must be used to make the stitch bond because they are thicker and better capable of withstanding the forces applied to the capillary during stitch bonding. With a capillary having the above described shape, when bonds are to be made concurrently at adjacent bond location, the adjacent bonds are made with the capillaries rotated by 90.degree. relative to each other so that the circular portion of one capillary fits into but is separated from the waist portion of the adjacent capillary. As is apparent, the bond spacing can be materially decreased with the above described technique. After ball bonding one end of the wire extending from the capillary, the capillary with wire therein is moved to the next bonding location during which time the wire is passed through the capillary. The other end of the wire just bonded is then stitch bonded at the next bonding location using the thicker portions of the capillary to make the stitch bond.
However, often, due to the yet smaller dimensions now being introduced and contemplated for the future, the direction of capillary travel from one bonding location to the next bonding location can still be presented with an impediment from the circular portions of the "8"-shape. This impediment can again be caused by the wires extending from an adjacent bond location in that the "8"-shaped capillary may strike the adjacent wires when travelling to the next bond location. It is necessary that the capillary not come in contact with the adjacent bond just made or being made or with the wire extending therefrom in order to avoid damage to one of the bonds or wires in question. This is often a problem due to the close proximity of adjacent bonds.
One way to minimize this problem is to further decrease the dimensions of the stitch face of the capillary. However this approach presents the problem of capillary breakage and poor stitch bond strength when forming the stitch bond as discussed above with the prior art. It is therefore apparent that a different solution to this problem is necessary.